1. Field of the Invention
The present invention relates to a vibration reducing technique of a power tool, such as a hammer and a hammer drill, which linearly drives a tool bit.
2. Description of the Related Art
Japanese non-examined laid-open Patent Publication No. 2004-154903 discloses an electric hammer having a vibration reducing mechanism. This known electric hammer has a dynamic vibration reducer as a means for reducing vibration caused in the axial direction of the hammer bit during hammering operation. The dynamic vibration reducer has a weight that can linearly move under a biasing force of an elastic element, and the dynamic vibration reducer reduces vibration of the hammer during hammering operation by the movement of the weight in the axial direction of the tool bit.
In the known dynamic vibration reducer, the weight has a long columnar shape having a large-diameter portion and a small-diameter portion which are integrally connected to each other. Further, a coil spring is disposed on the outer periphery of the small-diameter portion. With this construction, the movement of the weight in the axial direction of the tool bit can be stabilized, while maintaining the mass of the weight.
In order to realize a lower-vibration hammer, it is effective to improve the vibration reducing performance of the dynamic vibration reducer, or to increase the vibration reducing power (power acting in a direction opposite to the direction of vibration) of the dynamic vibration reducer. In the known dynamic vibration reducer, however, the coil spring having a lower density than the weight (due to existence of axial voids between adjacent coil parts) is disposed on the outer peripheral side of the weight. Thus, in terms of ensuring the mass of the weight, it has a poor space utilization efficiency. If the weight is increased in mass with the aim of improving the vibration reducing performance, the dynamic vibration reducer will be increased in size and thus require a larger installation space in the power tool.